Furnace method for reheating billets or slabs

ABSTRACT

A slab-reheating pusher furnace has a preheating zone followed by a major soak zone. A burner in the preheating zone is arranged so that the flame travels in the same direction as the slabs and a burner in the soak zone is arranged so that the flame travels in the opposite direction. The length of the preheating zone is greater than the length of the soak zone and various other dimensions of the furnace are specified which result in an increased throughput.

United States Patent James Alan Costick Whitstable;

William Robert Laws, Worcester Park; Frank Michael Salter, Chatham, all of [72] Inventors England [21] Appl. No. 4,517 [22] Filed Jan. 21, 1970 [45] Patented [73] Assignee Nov. 30, 1971 The British Iron and Steel Research Association London, England [32] Priority Feb. 4, 1969 [3 3] Great Britain [31 5,966/69 [54] FURNACE METHOD FOR REHEATING BILLETS 01R SLABS 2 Claims, 9 Drawing Figs.

[51 Int. Cl l 27b 9/02 [50] Field of Search 263/6 R, 6 B. 36. 52

[56] References Cited UNITED STATES PATENTS 1,476,142 12/1923 Bradshaw 263/6 3.305,!09 2/1967 Kerr 263/6 X Primary E.raminer-John J. Camby Allorney- Bacon 8: Thomas ABSTRACT: A slab-reheating pusher furnace has a preheating zone followed by a major soak zone. A burner in the preheating zone is arranged so that the flame travels in the same direction as the slabs and a burner in the soak zone is arranged so that the flame travels in the opposite direction. The length of the preheating zone is greater than the length of the soak zone and various other dimensions of the furnace are specified which result in an increased throughput.

PATENTEI] NUVSO l97| SHEET 3 0F 3 FIG. 8.

INVE N TOPS JAMES Hum 605 TICK W/LLl/IM Fewer Laws FkmvK M/CH/IEL 6m TEI? H TTORNE Y5 FURNACE METHOD FOR REHEATING BllLLETS R SLABS This invention relates to a furnace for reheating billets or slabs and is applicable to both top-fired and topand bottomfired furnaces.

It is an object of this invention to provide a furnace capable of having a high throughput rate of uniform temperature stock with low surface oxidation.

We have previously proposed a furnace with a preheating zone and a soak zone with a burner arranged in the preheating zone so that the burner flame travels in the same direction as the slabs or billets i.e. from the preheating zone to the soak zone. We have now found that the throughput of the furnace can be considerably improved if the furnace is appropriately dimensioned.

According to the present invention a furnace for reheating billets or slabs having a preheating zone, a major soak zone, supporting means for supporting slabs or billets during passage through the furnace, a burner arranged in the preheating zone so that the burner flame travels in the same direction as the slabs or billets, a burner arranged in the soak zone so that the burner flame travels in the opposite direction to that of the slabs or billem, a waste gas offtake disposed between the preheating and the soak zones, is characterized in that the preheating zone is greater in length than any subsequent heating zone. Preferably the ratio of the length of the preheating zone to the sum of the length of the preheating zone and major soak zone is two-thirds plus or minus percent.

With an arrangement according to the invention the heat flux distribution along the length of the furnace can be such that the heat imparted to a slab or billet is high initially and then falls off rapidly. This has the result that the temperature of the surface of a cold slab is quickly brought up to the desired level and this level is then maintained as the slab passes through the furnace while the temperature of the center of the slab rises to the desired level at which moment the slab is discharged. Thus slabs of different thicknesses can be passed through the furnace without a risk of overheating the thinner slabs. In conventional furnaces where the pattern of heat flux is such that maximum flux is towards the middle of the furnace thin slabs will overheat if the rate of passage through the furnace is governed by thick slabs or conversely the furnaces will only produce a small throughput if the thin slabs are not to overheat. A furnace according to the invention can heat both thick and thin slabs with a surprisingly increased throughput as compared with conventional furnaces. For example as compared with furnaces in current use of the same overall length the preferred furnace according to the invention may have a throughput which is greater by up to 50 percent or 60 percent.

The furnace is preferably arranged so that in use the maximum heat-flux density to the slab or billet surface in the preheating zone exceeds 250 kilowatts per square meter. It may exceed 300 kilowatts per square meter and may range up to 400 kilowatts per square meter or more.

Preferably the furnace is arranged so that in use the distribution of the heat flux to the billet or slab surface is such that the distance of the point of maximum heat flux from the preheating zone entry is less than one-third and preferably one sixth of the sum of the length of the preheating zone and major soak zone.

Preferably the furnace is arranged so that in use the quantity of heat supplied is greater in the first third, and preferably the first quarter, of the combined length of the preheating zone and major soak zone than in the rest of the furnace.

We have realized that this desirable heat distribution can best be obtained if, in addition to the relative lengths of the major zones specified above various other ratios of dimension are observed. This is because the radiation reaching the slabs or billets consists of radiation from the flames as well as radiation from the furnace structure. If the preheating zone flame is too long there will not be sufficient heat flux near the entry of the furnace and the slabs or billets will overheat during their passage through a subsequent part of the furnace. If the flame is too short too much heat will be supplied immediately after the entry. If the flame is too near the slabs there will be excessive radiation under the flame, but if the flame is too far from the slabs too much heat will radiate to the slabs at the later stages of their passage through the furnace. If the height of the zones above the slabs is too small there may be inadequate combustion, but if the height of the zones is too large, radiation will be diffuse and the slabs will not be heated rapidly at the beginning.

Preferably the ratio of the height of the preheating zone burner tip above the slab supporting means to the length of the preheating zone is one-sixth plus or minus 15 percent.

Preferably the furnace is arranged so that the ratio of the length of the preheating zone burner flame at its maximum firing rate to the sum of the length of the preheating and soak zones is one-third plus or minus 15 percent.

Preferably the ratio of the maximum height of the preheating zone above the supporting means to the length of the preheating zone is one-quarter plus or minus l5 percent.

Preferably the ratio of the maximum height of the soak zone above the supporting means to the length of the soak zone is one-third plus or minus 20 percent.

Preferably the axis of the preheating zone burner is inclined downwardly by an angle of 10 or less.

Conventional furnaces sometimes have a preheating zone followed by more than one further heating zones called tonnage zones or soak zones. With a furnace according to the present invention there are only two major heating zones above the slab supporting means namely the preheating zone and the major soak zone, through which the slabs pass in sequence, although an auxiliary soak zone can be provided if necessary to reduce skid marks on the slabs. The two major zones serve a different function from the auxiliary soak zone, because in the major zones the vertical temperature gradient of the slabs or billets is modified whereas the auxiliary soak zones, if it is provided, serves to reduce skid marks. ln the major zones probably at least percent of the fuel is burned. The effective heating length of the furnace is therefore the sum of the length of the preheating zone and major soak zone. As used in this specification the phrase the sum of the length of the preheating zone and major soak zone" does not include any auxiliary soak zone.

The effective heating length can be regarded as the distance between the end walls of the major zones. Although the length of the preheating zone is theoretically measurable from the end wall of the furnace to the dividing streamline in the offtake it can for practical purposes be measured to the centerline of the offtake. The height of the preheating zone burner tip and the maximum height of the zones can for practical purposes be measured from the slab-supporting means. The length of the preheating zone burner flame at its maximum firing rate can be regarded as the length from the burner tip to the point where the temperature of the flame is the mean between the maximum flame temperature and the ofltake waste gas temperature.

One of the problems associated with conventional furnaces is that there is usually a rise of pressure within the furnace from the exit to the entry resulting from the destruction of the input momentum flux of the burners. As a result waste gases leak continuously from the front end of the furnace under and around the charge end doors. Also, if the furnace is provided with a discharge ramp, the pressure of the bottom of the discharge ramp may be below atmospheric pressure with the result that cold air leaks into the furnace. This cold air oxidizes the hot steel causing loss in yield and also lowers the furnace gas temperatures. This problem is not completely solved by having the burner in the preheating zone opposed to the burner in the major soak zone since the momentum flux of the preheating burner is greater than the momentum flux of the major soak zone burner. However, we have found that this problem can be overcome by making use of the phenomenon that there is a pressure difference between opposite ends of nun-1....

the discharge ramp due to the column of hot gases contained along the ramp.

Thus according to a preferred feature of the invention the furnace includes a discharge ramp the height of which is so arranged in relation to the momentum fluxes of the burners at their maximum firing rate that the pressures at the entry of the furnace and at the bottom of the discharge ramp are substantially equal. Preferably the fumace is arranged so that the pressures at the entry of the furnace and at the bottom of the discharge ramp are substantially atmospheric.

IN THE ACCOMPANYING DRAWINGS FIG. 1 is a diagrammatic side view of a known five zone furnace,

FIG. 2 is a graph showing a typical heat flux distribution along the length of the known furnace of FIG. 1,

FIG. 3 is a graph of the pressure within the furnace of FIG. 1 plotted against the length of the furnace,

FIGS. 4 and 5 are diagrammatic side views respectively of a top-fired and a topand bottom-fired furnace according to the invention,

FIG. 6 is a graph showing a typical heat flux distribution along the furnace of FIG. 5,

FIG. 7 is a graph of the pressure within the furnace of FIG. 5 plotted against the length of the furnace,

FIG. 8 is a side view of another furnace according to the invention, and

FIG. 9 is section on the line IX--IX of FIG. 8.

The furnace shown in FIG. 1 is a known top and bottom fired five zone pusher furnace. The furnace has an entry 1 and exit 2 followed by a discharge ramp 10.

The preheating zones 3, the tonnage zones 4 and soak zone 5 each have a burner arranged so that the burner flame 6 travels in the opposite direction to that of the slabs or billets during their passage through the furnace. An offtake 7 for exhaust gases is disposed near the entry 1. The offtake can be controlled in size by closure 9. FIG. 2 shows that the heat flux distribution plotted in kilowatts per square meter of slab surface against the length of the furnace has a fairly constant level throughout the greater part of the length of the furnace and is everywhere less than 200 kilowatts per square meter.

FIG. 3 shows that the pressure of the gases within the furnace increases from exit 2 towards the entry 1. The values of the pressures are not given. By adjusting the closure 9 the pressure at the exit 2 can be arranged to be atmospheric but in that case the pressure at the bottom of the discharge ramp will be less than atmospheric because of the known effect of the bouyancy of the column of hot gases along the discharge ramp.

FIGS 4 and 5 show pusher furnaces for reheating billets or slabs according to the invention each having an entry 12 and exit 13. Each furnace has a preheating zone 14, a major soak zone 15, supporting means 16 for supporting slabs or billets during passage through the furnace, a burner 17 arranged in the preheating zone 14 so that the burner flame travels in the same direction as the slabs or billets, a burner 18 arranged in the major soak zone so that the burner flame travels in the opposite direction to that of the slabs or billets, and a waste gas offtake 19 disposed between the preheating and the soak zones l4, 15. The furnace shown in FIG. 5 differs from that shown in FIG. 4 in that it is top and bottom fired, i.e. there is a preheating zone 20 and a major soak 21 beneath the slab-supporting means 16.

The furnaces shown in FIGS. 4 and 5 are dimensioned so that the preheating zone is greater in length than any subsequent heating zone and more particularly as follows:

1. Lp/L=2/3:l5 percent 3. Hp/Lp=l/4:l5 percent 4. Hs/L.r=l/3i20 percent 6. Fp/L=l/3:l5 percent Where L the sum of the length of the preheating and major soak zones.

Lp length of the preheating zone.

Ls= length of the major soak zone.

Hp maximum height of the preheating zone.

Hs maximum height of the soak zone.

Hf height of the preheating zone burner tip from the slab supporting means.

Fp length of the flame of the preheating zone burner at its maximum firing rate.

The axis of the preheating zone burner is inclined towards the supporting means by an angle of less than 10.

FIG. 6 shows a typical heat-flux density distribution for the furnace of FIG. 5. The distance of the point 25 of maximum heat flux from the preheating zone entry 12 is less than onesixth of the sum of the length of the preheating zone 14 and major soak zone 15. The area under the curve in FIG. 6 is proportional to the quantity of heat supplied and it can be seen that the quantity of heat supplied is greater in the first third, and even in the first quarter, of the distance L than in rest of the furnace. The maximum heat-flux density represented by the point 25 exceeds 300 kilowatts per square meter. As is well-known, 1 watt is equal to l joule per second.

A comparison of FIG. 6 and FIG. 2 shows that the furnace according to the invention imparts much more heat to the slabs or billets initially than the conventional furnace and for the reasons giveh above this allows a higher throughput even when simultaneously heating both thick and thin slabs.

The furnace shown in FIG. 5 has a discharge ramp 27 beyond the exit 13 from the major soak zone 15. FIG. 7 shows a typical pressure distribution within the furnace. The pressure is atmospheric at the entry 12, then rises in the preheating zone 14. Along the major soak zone 15 the pressure drops, but because the momentum flux of the soak zone burner is less than the momentum flux of the preheating zone burner the pressure at the exit 13 from the major soak zone 15 is higher than atmospheric as indicated by point 28 in FIG. 7. There is always a pressure drop along the length of a discharge ramp because of the bouyancy effect of the hot gases and accordingly the height of the discharge ramp can be so arranged in relation to the momentum fluxes of the burners at their maximum firing rate that the pressures at the entry of the furnace and at the bottom of the discharge ramp are substantially both atmospheric. Thus cold air will not leak into the furnace to oxidize the slabs and hot gases will not escape during entry and discharge of slabs resulting in a loss of etficiency.

The furnace shown in FIGS. 8 and 9 is a preferred form of furnace and is shown in more detail. The dimensions marked have the same ratios as the dimensions of the furnaces shown in FIGS. 4 and 5. The furnace has a charging passage 30 through which slabs or billets are pushed with their axes extending transversely to the length of the furnace. The furnace is top and bottom fired so there are two preheating zones 31 and 32, and two major soak zones 33 and 34.

A skid system, for example as described in our British Pat. application, 7724/69 (P.583 supports the slabs or billets during passage through the furnace. The length of the preheating zone 31 is 17.5 meters, and the length of the major soak zone is 9.5 meters. The burner 36 in the preheating zone 31 is inclined downwardly towards the skid system 35 at an angle of 6". There is a burner 37 in the preheating zone 32, and burners 38 and 39 on the zones 33 and 34. The waste gas offtake 40 is disposed between the preheating zone 31 and the major soak zone 33.

An auxiliary soak zone 41 having a burner 42 is provided for ensuring that any skid marks on the slabs or billets are reduced. Less than 10 percent of the total fuel is burnt by the burner 42. There is a discharge ramp 43 provided with a discharge door 44. The height of the discharge ramp is l.6 meters and this has been calculated in relation to the momentum fluxes of the burners so that the pressures at the entry 30 and at the bottom of the discharge ramp 43 are atmospheric. The momentum fluxes of the burners 36 and 37 at maximum firing rate is 267 Newtons and the momentum flux of the burners 38, 39 and 42 is 31 Newtons. At the maximum firing rate the pressure at the offtake is 7.5 Newtons per square meter above atmospheric pressure.

The furnace described with reference to FIGS. 8 and 9 not only provides an increased throughput when compared with conventional furnaces but also the pressures at entry and exit are balanced resulting in greater yield and heat conservation.

We claim:

1. A process for reheating billets or slabs by moving the slabs or billets through a furnace while supported on supporting means, comprising heating the slabs or billets by passing them through a preheating zone heated by a burner having a flame travelling in the same direction as the slabs or billets, then further heating the slabs or billets by passing them through a major soak zone heated by a burner having a flame travelling in the opposite direction to that of the slabs or billets, the preheating zone being greater in length than the major soak zone, supplying a greater quantity of heat to the billets or slabs in the first quarter of the combined length of the preheating zone and major soak zone than in the rest of the furnace, venting the furnace through a waste gas offtake located between the preheating and soak zones, and then discharging the slabs or billets down a discharge ramp and through a door at the bottom of the ramp, the height of the discharge ramp being arranged in relation to the momentum fluxes of the burners at their maximum firing rate so that the pressure in the furnace rises from the entry of the furnace along the length of the preheating zone, decreases along the length of the major soak zone, and further decreases along the length of the discharge ramp and is substantially equal at the bottom of the ramp to the pressure at entry.

2. A process as claimed in claim 1 in which the pressures at the entry and at the bottom of the ramp are substantially atmospheric.

* l II 

1. A process for reheating billets or slabs by moving the slabs or billets through a furnace while supported on supporting means, comprising heating the slabs or billets by passing them through a preheating zone heated by a burner having a flame travelling in the same direction as the slabs or billets, then further heating the slabs or billets by passing them through a major soak zone heated by a burner having a flame travelling in the opposite direction to that of the slabs or billets, the preheating zone being greater in length than the major soak zone, supplying a greater quantity of heat to the billets or slabs in the first quarter of the combined length of the preheating zone and major soak zone than in the rest of the furnace, venting the furnace through a waste gas offtake located between the preheating and soak zones, and then discharging the slabs or billets down a discharge ramp and through a door at the bottom of the ramp, the height of the discharge ramp being arranged in relation to the momentum fluxes of the burners at their maximum firing rate so that the pressure in the furnace rises from the entry of the furnace along the length of the preheating zone, decreases along the length of the major soak zone, and further decreases along the length of the discharge ramp and is substantially equal at the bottom of the ramp to the pressure at entry.
 2. A process as claimed in claim 1 in which the pressures at the entry and at the bottom of the ramp are substantially atmospheric. 